Report effectful top-level exprs

crates/compiler/constrain/src/expr.rs

@@ -1891,7 +1891,7 @@ fn constrain_call_fx(
             ))
         }
         None => constraints.push_expected_type(ForReason(
-            Reason::CallInTopLevelDef,
+            Reason::CallInTopLevel,
             // top-level defs are only allowed to call pure functions
             constraints.push_variable(Variable::PURE),
             region,

crates/compiler/load/tests/test_reporting.rs

@@ -14824,6 +14824,44 @@ All branches in an `if` must have the same type!
     "###
     );
 
+    test_report!(
+        effect_in_top_level_value_def,
+        indoc!(
+            r#"
+            app [main!] { pf: platform "../../../../../examples/cli/effects-platform/main.roc" }
+
+            import pf.Effect
+
+            hello =
+                Effect.putLine! "calling hello!"
+                "hello"
+
+            main! = \{} ->
+                Effect.putLine! hello
+            "#
+        ),
+        @r###"
+    ── EFFECT IN TOP-LEVEL in /code/proj/Main.roc ──────────────────────────────────
+
+    This top-level expression calls an effectful function:
+
+    6│      Effect.putLine! "calling hello!"
+            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+    However, only functions are allowed to be effectful. This limitation
+    ensures that importing a module never produces a side effect.
+
+    Tip: If you don't need any arguments, use an empty record:
+
+        askName! : {} => Str
+        askName! = \{} ->
+            Stdout.line! "What's your name?"
+            Stdin.line! {}
+
+    This will allow the caller to control when the effect runs.
+    "###
+    );
+
     test_report!(
         aliased_fx_fn,
         indoc!(

crates/compiler/lower_params/src/type_error.rs

@@ -186,7 +186,7 @@ fn remove_for_reason(
         | Reason::CrashArg
         | Reason::ImportParams(_)
         | Reason::CallInFunction(_)
-        | Reason::CallInTopLevelDef
+        | Reason::CallInTopLevel
         | Reason::Stmt
         | Reason::FunctionOutput => {}
     }

crates/compiler/types/src/types.rs

@@ -3450,7 +3450,7 @@ pub enum Reason {
     },
     Stmt,
     CallInFunction(Option<Region>),
-    CallInTopLevelDef,
+    CallInTopLevel,
     FloatLiteral,
     IntLiteral,
     NumLiteral,